In recent years, a base station having a small communication coverage area of up to several tens of meters in radius has been developed, and is referred to as a femto base station. A base station having a wide communication coverage area (for example, ranging from several hundred meters to a dozen or so kilometers in radius) is referred to as a macro base station.
By installing such a femto base station in a place where radiowaves from the macro base station are difficult to reach, the communication coverage area of a mobile station such as a mobile phone can be expanded. Further, when a large number of femto base stations are installed, the number of mobile stations connected to one particular base station can be reduced. Since this serves to increase the amount of radio resources to be allocated to each mobile station, the communication speed increases, and as a result, better communication service can be provided to the user of the mobile station.
Further, by installing a femto base station at a user's premises and allocating a private channel to a specific user, it is possible to provide a communication service that allows the specific user to monopolize that channel.
When installing a femto base station, the radio frequency that the femto base station uses is chosen to be as different as possible from the radio frequency used by any macro base station located in the neighborhood of the femto base station, in order to prevent the occurrence of radiowave interference in the area where the femto base station is installed. However, in a place such as an apartment complex where many femto base stations are installed within a limited area, it may be difficult to set the radio frequency of each femto base station so as to avoid possible radiowave interference. Furthermore, depending on the congestion in a macro cell defined by the macro base station to cover the area of the femto base station, radiowave interference may easily occur. Further, if the radio frequency of any given femto base station is chosen so as not to cause radiowave interference, there are still cases where radiowave interference occurs during the operation of the femto base station. For example, when the mobile station currently radio-linked to the macro base station moves around or moves toward the femto base station, the radiowave conditions may change. In such cases, radiowave interference occurs at the mobile station or at one or the other of the base stations.
If such radiowave interference occurs, the quality of the radiowave received at the mobile station or at the base station degrades, and the mobile station or the base station may not be able to reproduce the received radiowave signal. This may result in a disconnection of the radio link between the mobile station and the base station, or may reduce communication throughput or cause a degradation in the quality of the voice received by the mobile station.
In view of the above, technologies for reducing such radiowave interference have been proposed in the prior art (for example, refer to Japanese Laid-open Patent Publication No. H09-247079). In one such prior art example, when radiowave interference occurs at a mobile station connected to a micro cell base station due to a radiowave being emitted from another mobile station connected to a macro cell base station, the mobile station experiencing the interference is handed off to a frequency band different from the currently used frequency band. Alternatively, the mobile station is handed off between the macro cell defined by the macro base station and the micro cell defined by the micro base station.